Reorganize source tree: kernel/, v3/, v4/ split and board infrastructure

Source tree reorganization:
- Move StarForth v3 engine to v3/ (src/, include/, Makefile)
- Move kernel to kernel/ (src/, include/, linker/, Makefile)
- Create v4/ skeleton for F18-ISA golden model (DECOMPOSITION.md, JUSTIFICATION.md)
- Move FABRIC-0..4.md to docs/fabric/
- Move ONTOLOGY.md and ROADMAP.md to docs/

Board infrastructure:
- Add boards/ser5/, boards/raspi/, boards/milkv/, boards/zynq7020/
- Each board has board.mk (ISA, CPU flags, boot recipe) and README.md
- Root Makefile becomes thin dispatcher: boot_image, all, clean, docs take TARGET
- make boot_image TARGET=SER5|RASPI|MILKV builds one GPT/MBR image per board
- ZYNQ7020 target exists but stops with clear error (ARMv7 port not built yet)
- scripts/mkdiskimage.sh builds disk images for all boards

Docs pipeline:
- docs/book/ with LaTeX master (main.tex) and Makefile
- pandoc converts Markdown to LaTeX at build time
- Two Lua filters: table-widths.lua (wide tables wrap), code-breaks.lua (inline code breaks)
- make docs builds single PDF (754 pages, 0 missing characters)
- make docs TARGET=<board> adds board appendix
- build/docs/<book|board>/meta.tex stamps git commit into PDF

Bug fixes:
- 42 include paths that only worked by accident now use correct relative paths
- clang-18 hardcode replaced with configurable CC variable (fixed aarch64 build)
- Pi 5: kernel_2712.img linked at 0x80000, .bss zeroed, memory reserved
- Doxyfile, .clang-tidy, README.md, Kconfig paths updated

Verified:
- Hosted v3 build passes 1012 tests, 0 failures
- SER5 image boots in QEMU (OVMF), POST passes, K exact (65536 = Q48_ONE)
- Milk-V image boots in QEMU (OpenSBI + U-Boot + bootefi), POST passes
- make clean TARGET=<board> removes only that board and its ISA objects
- make all builds all boards, hosted v3, and docs in one run

Co-authored-by: Junie <junie@jetbrains.com>

v3/src/word_source/include/log_words.h

@@ -0,0 +1,62 @@
+/*
+  StarForth — Steady-State Virtual Machine Runtime
+
+  Copyright (c) 2023–2025 Robert A. James
+  All rights reserved.
+
+  Licensed under the StarForth License, Version 1.0
+*/
+
+#ifndef LOG_WORDS_H
+#define LOG_WORDS_H
+
+#include "vm.h"
+
+/**
+ * @defgroup log_words Log Words
+ * @{
+ *
+ * @brief FORTH words for log-level control and structured string emission.
+ *
+ * @par Level constants ( -- n )
+ * Push the corresponding LogLevel integer onto the stack.
+ *   LOG-ERROR ( -- 0 )
+ *   LOG-WARN  ( -- 1 )
+ *   LOG-INFO  ( -- 2 )
+ *   LOG-TEST  ( -- 3 )
+ *   LOG-DEBUG ( -- 4 )
+ *
+ * @par LOG-LEVEL! ( n -- )
+ * Set the current log level to n.  Values outside 0–4 clamp to LOG-DEBUG.
+ *
+ * @par LOG-LEVEL@ ( -- n )
+ * Push the current log level.
+ *
+ * @par LOG-INFO" ( "ccc<quote>" -- )   IMMEDIATE
+ * @par LOG-WARN" ( "ccc<quote>" -- )   IMMEDIATE
+ * @par LOG-ERROR" ( "ccc<quote>" -- )  IMMEDIATE
+ * @par LOG-TEST" ( "ccc<quote>" -- )   IMMEDIATE
+ * @par LOG-DEBUG" ( "ccc<quote>" -- )  IMMEDIATE
+ * Parse the string literal up to the closing double-quote and emit it
+ * via log_message() at the corresponding level.  Works in both interpret
+ * and compile mode (like .").  The level filter is applied at runtime so
+ * DEBUG messages are free when the level is INFO.
+ *
+ * @par LOG-INFO-STR  ( c-addr u -- )
+ * @par LOG-WARN-STR  ( c-addr u -- )
+ * @par LOG-ERROR-STR ( c-addr u -- )
+ * @par LOG-TEST-STR  ( c-addr u -- )
+ * @par LOG-DEBUG-STR ( c-addr u -- )
+ * Consume a counted string from the stack (as produced by S") and emit
+ * it via log_message() at the corresponding level.
+ *
+ * @par Runtime words (not for direct use)
+ * (do-log-error)  (do-log-warn)  (do-log-info)
+ * (do-log-test)   (do-log-debug)
+ * Threaded-code runtime helpers compiled by the LOG-*" immediates.
+ * @}
+ */
+
+void register_log_words(VM *vm);
+
+#endif /* LOG_WORDS_H */